MOSFET Datasheet Confusion

Question

I am working on a 12v automotive circuit and I needed to switch a 12v solenoid with Pi Pico which has a 3.3v logic.

Initially I picked an IRLB8721 MOSFET which claims a 2.5v V(gs) thinking it would work. To my surprise it didn't so I went back to the datasheet and rechecked this chart, showing that at 3.0v V(gs) and 12v (Vds) it should be able to flow over 3 amps

I did some searching and came across multiple posts where people have pointed to fig 12 below saying it doesn't really work below 2.5v

I am having a hard time reconciling all that info. Feels like each one contradicts the others. Would someone please help clarify how to read these charts ?

UPDATE
Not this this directly answers the question, but I learned that the MOSFETs I purchased were counterfeit. Part of my confusion is that the Vds x Id chart did not reflect reality. The chart shows that the MOSFET should be at least partially on at 3v while in reality it flows 0mA.

Answer 1

For one thing, these graphs are typical values, not guaranteed values. Threshold voltage is one of the more variable parameters in a FET, and it's not unusual for it to differ by hundreds of mV between lots.

But fundamentally, you seem to be falling for a trap that a lot of hobbyists fall into, of assuming that a FET is 100% on when the gate-source voltage exceeds its threshold voltage and 100% off when it doesn't. While it is true that it's almost completely off when Vgs < Vth, you need to exceed Vth by at least a handful of volts for it to be anything approaching fully on. The IRLB8721 is not designed to be used with gate voltages below 4.5 V, the lowest gate voltage for which an Rdson is specified.

You're almost there checking that first graph, but you've missed the fact that the solenoid requires some voltage as well--you don't have Vds = 12 V if you want to actually power the solenoid. Without data on the specific solenoid, I can't tell you exactly what voltage/current is required to actuate it, but it's apparently more than it's getting when you drive the gate at 3.3 V.

Answer 2

Based on a comment I assume 100mA is required when the solenoid is energized.

I am having a hard time reconciling all that info. Feels like each one contradicts the others. Would someone please help clarify how to read these charts ?

Figure 1 shows the curves for the drain current \$I_D\$ vs the drain-source voltage \$V_{DS}\$ (not the applied voltage) with the gate source voltage as a parameter. The circuit has two operating points.

1. ON where \$V_{DS}\approx 0\$ and \$I_D\approx 100mA\$.
2. OFF where \$V_{DS}=12V\$ and \$I_D=0A\$.

Neither of these points can be plotted on the Figure 1 diagram, and so this transistor is not suitable for your application.

May I suggest studying load-lines. Perhaps this Wikipedia page will help. Figure 1 uses logarithmic axes so a load-line is difficult to use here. The points should fall on the vertical and horizontal axis respectively to use a load line.

As noted in the OP, Figure 12 in the datasheet confirms that operating with \$V_{GS} < 4V\$ results in a very high \$R_{DSon}\$. Notice also that Figure 12 was created for \$I_D=31A\$. So again this transistor is not suitable.

Figure 6 shows the gate charge required to turn on the FET. The horizontal line is called the Miller plateau. \$R_{DSon}\$ is decreasing rapidly (FET turning ON) as charge is applied to the gate-drain capacitance in this region. While this is more a dynamical chart (PWM for example), it clearly shows that \$V_{GS}>\approx 3.8V\$ is desirable, further confirming that this transistor is not suitable for this application.

I won't go into the details of this chart. It is very interesting though and worthy of study.

Answer 3

The second graph you showed is annotated with \$I_D=31A\$, which is well beyond your own requirement, and only represents behaviour along that line on the \$I_D\$ vs. \$V_{DS}\$:

The graph of \$R_{DS(ON)}\$ isn't very useful here, except in the sense that it does suggest that the device is expected to be used in applications demanding far greater drain current than you will be using.

at 3.0v V(gs) and 12v (Vds) it should be able to flow over 3 amps

You have forgotten than when the transistor is on, you require it to have almost no voltage across it, \$V_{DS} \approx 0V\$:

^{simulate this circuit – Schematic created using CircuitLab}

The relay coil should have all 12V, and therefore, that 12V, 3A point is irrelevant for your calculations.

Rather, you will be operating the device near the left edge of the graph of \$I_D\$ vs. \$V_{DS}\$, near \$V_{DS}=0.1V\$, for which the appropriate line (bottom) shows \$I_D \approx 1A\$.

I seriously doubt that anything strange happens further to the left than shown on that graph, and I would expect this MOSFET to maintain low enough \$R_{DS}\$ that \$V_{DS}\$ remains below 0.1V for all currents below 1A. For this reason, I too would expect this device to work in your application.

Despite that belief, I wouldn't recommend making such extrapolations. That the graph doesn't include conditions under which you intend to operate is an indication that this device probably isn't right for you.

I remind you also that the quoted worst-case \$V_{GS(TH)}=2.35V\$ is valid only for \$I_D=25\mu A\$, and for \$V_{DS}\$ significantly greater than zero, so this value is not a good indicator of what gate potential will pass hundreds of milliamps of drain current.

Still, why doesn't it work? I think it should. Perhaps it's damaged, which is easy enough to do with imprudent handling. The gate isn't protected against ESD like most CMOS integrated circuit inputs, so if you don't take care to avoid ESD, then you could easily blast a hole in the gate before you even switch the circuit on.

Or, perhaps you got yourself a counterfeit device.

Aside from buying another one (from a reputable source), I would recommend testing the device in some kind of test rig, to obtain your own graphs, and see how they agree or differ from the datasheet's claims. That's what I would do.

Answer 4

Instead of playing roulette with uncertainties and variability of bare MOSFETS, you can use an industrial-grade solution called "power distribution switches and load drivers", check, for example, DigiKey. Something like BTS7002:

These devices do cost more, but they provide reliable switching of many kinds of loads with lots of controls and sensors, and they are "automotive grade".